1. Field of the Invention
The present invention relates to a liquid crystal display device and a driving circuit for a liquid crystal panel with a memory effect, and particularly to a liquid crystal display device and a driving circuit for its liquid crystal panel with a memory effect in which a memory effect presented by two stable states of a liquid crystal having a memory effect is utilized to enable operation at a low voltage and reduce the power consumption.
2. Description of the Related Art
As a display device of a personal digital assistant in which the display screen is not often switched, such as used in an electronic book or electronic newspaper which has recently received much attention, a liquid crystal panel with a memory effect using a liquid crystal having a memory effect has drawn attention. Having a memory effect means that a display state can be maintained even during application of no voltage. Using the characteristics enables reduction in the power consumption of the liquid crystal display device. A ferroelectric liquid crystal, a cholesteric liquid crystal, and so on are known as materials of the liquid crystal for use in the liquid crystal panel with a memory effect.
Such a liquid crystal panel with a memory effect has a liquid crystal having a memory effect having at least two stable sates between a pair of substrates (glass substrates) which have scanning electrodes and signal electrodes on their opposed surfaces, respectively.
FIG. 7 is a plane view of portions of the scanning electrodes and signal electrodes as seen from a direction perpendicular to the substrate surface of the liquid crystal panel, in which TP1 to TP4 are scanning electrodes and SG1 to SG4 are signal electrodes. A liquid crystal having a memory effect exists between the scanning electrodes and the signal electrodes, and portions where the scanning electrodes TP1 to TP4 are opposed to the signal electrodes SG1 to SG4 with the liquid crystal having a memory effect intervening therebetween (portions where the scanning electrodes TP1 to TP4 overlap the signal electrodes SG1 to SG4 in FIG. 7) form pixels Pix, respectively.
Electro-optic effects of the ferroelectric liquid crystal used as the liquid crystal having a memory effect will be described now. FIG. 8 and FIG. 9 are explanatory views each showing the relation between a molecular long axis direction of the ferroelectric liquid crystal and an electric field. These drawings, which schematically show the liquid crystal molecule when a liquid crystal panel 1 is seen from a viewer side, are used to describe an average molecular long axis direction of the ferroelectric liquid crystal.
For example, when an electric field E occurs from the front side to the rear side in a direction perpendicular to the paper surface of the drawing as shown in FIG. 8, a liquid crystal molecule LCM is in a first ferroelectric state. An average molecular long axis direction M in that state is stable inclined counterclockwise by an angle θ1 with respect to an alignment axis OA of an alignment film. On the other hand, when the electric field E occurs from the rear side to the front side of the paper surface of the drawing as shown in FIG. 9, the liquid crystal molecule LCM is in a second ferroelectric state. The average molecular long axis direction M in that state is stable inclined clockwise by an angle θ2 with respect to the alignment axis OA.
In other words, the liquid crystal molecule LCM transfers on the side surface of a cone shape drawn with the molecular long axis direction M as a moving straight line. Further, the sum of the angle θ1 and the angle θ2 (θ1+θ2) is an angle between the average molecular long axis direction of the liquid crystal in the first ferroelectric state and the average molecular long axis direction of the liquid crystal in the second ferroelectric state, that is, a central angle of the cone (that is a cone angle) θ.
FIG. 10 is an explanatory view showing the relation between the molecular long axis direction of the ferroelectric liquid crystal and absorption axes of a pair of polarizing plates disposed outside a pair of substrates having the liquid crystal therebetween. As shown in this drawing, in the case using the ferroelectric liquid crystal, a first polarizing plate and a second polarizing plate are typically arranged such that a polarization axis P1 of the first polarizing plate and a polarization axis P2 of the second polarizing plate form an angle of almost 90° C. (to be perpendicular). Further, one of the polarization axes is aligned with the molecular long axis direction M when the ferroelectric liquid crystal is in the first or second ferroelectric state (in the example shown in FIG. 10, the molecular long axis direction M being aligned with the polarization axis P1).
As described above, in the ferroelectric state in which the molecular long axis direction M is aligned with the polarization axis, the transmittance decreases, thereby enabling a black image. When the electric field E is inversely directed, the liquid crystal molecule LCM moves with the alignment axis OA as a symmetrical axis to increase in transmittance, thereby enabling a white image.
The polarizing plate used here is an absorption-type polarizing plate which absorbs linearly polarized light whose polarization direction is parallel to its absorption axis and transmits linearly polarized light whose polarization direction is parallel to its polarization axis (transmission axis) perpendicular to the absorption axis.
FIG. 11 is a characteristic chart showing the relation between the voltage applied to a liquid crystal panel in which the ferroelectric liquid crystal and the pair of polarizing plates are arranged as described above, the transmittance, and two stable states of the ferroelectric liquid crystal.
The ferroelectric liquid crystal has two stable states, which are switched by applying a positive or negative voltage exceeding a threshold voltage Vt or −Vt, so that the first ferroelectric state (ON state) or the second ferroelectric state (OFF state) can be selected depending on the polarity of the applied voltage. More specifically, during the initial (application of no voltage) period, the ferroelectric liquid crystal exits stabile in the first or the second ferroelectric state. For example, when the applied voltage exceeds the threshold voltage Vt on the positive side while the ferroelectric liquid crystal is stabile in the second ferroelectric state (the black image state with a low transmittance), the ferroelectric liquid crystal is brought into the first ferroelectric state (the white image state with a high transmittance). Even if the applied voltage is gradually decreased from that state, the first ferroelectric state is maintained.
However, when the applied voltage exceeds the threshold voltage −Vt on the negative side, the liquid crystal is brought into the second ferroelectric state (the black image state with a low transmittance). Even if the applied voltage is gradually increased from that state, the second ferroelectric state is maintained. As is clear from the characteristic chart, the liquid crystal panel using the ferroelectric liquid crystal can maintain the transmittance, that is, the display state even during application of no voltage, that is, while the power consumption is zero. The characteristics mean having a memory effect.
Incidentally, the liquid crystal panel in which the pixels Pix are formed in a matrix form as shown in FIG. 7 typically performs display in a time division driving method. More specifically, a scanning voltage is applied from a scanning electrode driving circuit (not shown) sequentially to the scanning electrodes TP1 to TP4 line by line, for example, to TP1, TP2, and so on, in synchronization with which, a signal voltage is applied from a signal electrode driving circuit (not shown) to signal electrodes SG1 to SG4 in a parallel manner. Note that the signal voltage is outputted in a waveform corresponding to image data to be displayed at each of the pixels Pix.
Further, a pair of polarizing plates (not shown) are arranged outside the liquid crystal panel such that their absorption axes are in a crossed-Nicols state so as to create the white image in the above-described ON state and the black image in the OFF state.
Next, a conventional driving method for bringing the pixels in such a ferroelectric liquid crystal panel into the white image or the black image will be described using FIG. 12. FIG. 12 shows a driving voltage waveform and a transmittance curve of a typical ferroelectric liquid crystal panel when a pixel Pix (1, 1) at the first row and first column in FIG. 7 is brought into the white image ON (W) and the black image OFF (B). To bring the pixel Pix (1, 1) at the first row and first column shown in FIG. 7 into the white image, during a scanning period (1 frame=F1) for displaying one screen, a reset period RS is set at the first portion, and a selection period SE for determining the display state and a non-selection period NSE for maintaining the display state are set thereafter.
During the reset period RS, bipolar pulses of voltages ±VRT are outputted as the scanning voltage to the scanning electrode TP1. Further, bipolar pulses of voltages ±VRS are outputted as the signal voltage to all of the signal electrodes SG1 to SG4. Thereby, a voltage of a composite voltage waveform made by combining the signal voltage waveform and the scanning voltage waveform is applied to the pixel Pix (1, 1) during the reset period RS, so that reset pulses of the voltages (VRT+VRS) and −(VRT+VRS) are applied as the composite voltage TS (1, 1). As for the transmittance, as shown at TV (1, 1), the pixel Pix (1, 1) is brought into the first ferroelectric state, that is, the white image with a high transmittance during the first half of the reset period RS because the positive voltage exceeding the threshold voltage Vt on the positive side described with FIG. 11 is applied, whereas the pixel Pix (1, 1) is brought into the second ferroelectric state, that is, the black image with a low transmittance during the second half of the reset period RS because the negative voltage exceeding the threshold voltage −Vt on the negative side.
Subsequently, during the selection period SE, zero and bipolar pulses at −VS and +VS are applied as the scanning voltage to the scanning electrode TP1, and zero and bipolar pulses at +VD and −VD being data voltages are applied to the signal electrode SG1. Thereby, the voltages of voltages zero, −(VS+VD), and (VS+VD) as selection pulses are applied between the scanning electrode TP1 and the signal electrode SG1 as the composite voltage TS (1, 1). Since the last voltage (VS+VD) exceeds the threshold voltage Vt on the positive side described with FIG. 11, the second ferroelectric state is changed to the first ferroelectric state and the transmittance shown at TV (1, 1) increases to thereby select the white image.
During the non-selection period NSE, the voltage of the scanning voltage applied to the scanning electrode TP1 is zero, and the signal voltage in a pulse waveform composed of the voltages zero and +VD and −VD being the data voltages is applied to the signal electrode SG1. The pulse shown by a square in the drawing is a pulse composed of the voltages zero, +VD, and −VD, and is composed of three pulses here. These may be, for example, three pulses of the voltages zero, +VRS, and −VRS similar to the reset voltage, or may be applied in another order.
During the non-selection period NSE, the signal voltage is reflected, as it is, on the composite voltage TS (1, 1), so that the voltages at the voltages zero, −VD, and +VD as holding pulses are applied between the scanning electrode TP1 and the signal electrode SG1. Since the absolute value of any of the voltages is smaller than the threshold voltage Vt or −Vt, the ferroelectric state determined during the selection period SE, that is, the transmittance is maintained, maintaining the white image.
As described above, in the conventional driving method, the driving voltage is composed of the bipolar reset pulses, the bipolar selection pulses and holding pulses, and requires nine level values (zero, ±VS, ±VD, ±VRS, and ±VRT). Further, because of bipolar pulses, the peak-peak value (±(VRT +VRS) in FIG. 12) needs to be twice the voltage to which the liquid crystal reacts.
As described above, pulse voltages at many values have conventionally been required to drive the liquid crystal panel with a memory effect, leading to complicated configurations of the scanning electrode driving circuit for outputting the scanning voltage and the signal electrode driving circuit for outputting the signal voltage (respective driver ICs), and increased cost.
Hence, to decrease the load on the scanning electrode driving circuit and the signal electrode driving circuit (driver ICs), a method is proposed in which respective independent voltage converting means are provided separately from the aforementioned driving circuits so as to vary the driving voltages to be applied to the scanning electrodes and the signal electrodes of the liquid crystal panel respectively, such as found in JP 2001-42812A. The liquid crystal element with a memory effect disclosed therein uses a cholesteric liquid crystal or a chiral nematic liquid crystal as the liquid crystal material and employs a configuration in which three display layers are stacked in the thickness direction.
Further, as found, for example, in JP 63-212921A, there also is a proposed liquid crystal display device in which the kinds of voltage level values of the driving voltages outputted by the scanning electrode driving circuit and the signal electrode driving circuit (driver ICs) are reduced, and both the scanning voltage waveform and the signal (data) voltage waveform are made unipolar.
As described above, time division drive the matrix-type liquid crystal panel which uses the ferroelectric liquid crystal having an operation mode with a memory effect and includes the scanning electrodes and the signal electrodes, the driving voltage requires many voltage level values because the scanning voltage to be applied to the scanning electrode is composed of the bipolar reset pulses and selection pulses and the signal voltage to be applied to the scanning electrode is composed of the bipolar reset pulses, selection pulses, and holding pulses in one scanning period (1 frame). Further, because of bipolar pulses, the peak-peak value need to be twice the voltage to which the liquid crystal reacts, and a driver IC with a high withstand voltage is required especially for driving the scanning electrodes, bringing about a problem of the IC being increased in chip size and price.
In the liquid crystal display device described in the above-described JP 2001-42812A, the scanning voltage and the signal (data) voltage are also formed by combining positive and negative voltages at many different level values. To this end, respective independent voltage converting means are provided separately from the driving circuits (driver ICs) and a high withstand voltage switch is used for switching the driving voltage, resulting in increased cost.
In the liquid crystal display device described in the above-described JP 63-212921A, both the driving voltage waveforms outputted by the scanning electrode driving circuit and the signal electrode driving circuit (driver ICs) are unipolar, and the kinds of required voltage level values are also reduced. However, voltage levels at five values, that is, 0, V, ½V, ¾V, and ¼V are still required, and both of the waveforms of the scanning voltage and the signal voltage are complicated, resulting in increased cost.